System for the removal of particulate matter and noxious compounds from engine exhaust gas

ABSTRACT

System for the removal of noxious compounds and particulate matter from exhaust gas of a compression ignition engine comprising a three way catalyst unit having an NH3-SCR activity, an ammonia oxidation activity and an adsorption activity of volatile vanadium and tungsten compounds volatilized off an upstream SCR active catalyst.

The present invention relates to a system for the removal of volatileorganic compounds, particulate matter and nitrogen oxides (NOx) fromexhaust gas of a compression ignition engine.

The exhaust system of modern vehicles with lean burning engines istypically equipped with an oxidation catalyst, a particulate filter anda catalyst for the selective reduction of NOx (SCR) in presence of areduction agent.

Oxidation catalysts being active in the oxidation of volatile organiccompounds and carbon monoxide and SCR catalysts are known in the art anddisclosed in numerous publications.

Typically employed particulate filters (DPF) in diesel exhaust gascleaning systems, are wall flow filters with a plurality if inlet andoutlet channels. The inlet channels are closed at their outlet side andthe outlet channels are closed at their inlet side, so that the gasflowing into the filter is forced through porous walls defining thechannels, whereby particulate matter is filtered off the gas.

To meet future emission regulations for diesel passenger cars and trucksrequires usage of both diesel particulate filter (DPPF) technology andNOx reduction catalyst. Due to its potential for fuel optimization andhigh efficiency in NOx removal, selective catalytic reduction usingammonia as a reductant (NH3-SCR) is presently the preferred technologyfor NOx reduction.

The SCR catalyst can be arranged as a separate unit upstream and/ordownstream the DPF. It has also been suggested in the art providing theDPF with an SCR catalyst to obtain more compact cleaning systems.

Catalysts for use in ammonia SCR are well known in the art. Of those,catalysts based on V₂O₅ and WO₃ supported on a TiO₂ carrier provide afundamental solution to effectively reduce NOx emissions from Dieselfueled vehicles by means of the Selective Catalytic Reduction (SCR) withammonia. Compared to alternative strategies for NOx emission controllike exhaust gas recirculation (EGR) and zeolite-based catalysts, agreat advantage of vanadium-based SCR catalysts is their SCR efficiency,robustness to sulfur and their price.

When operating a cleaning system with a DPF, particulate matter trappedin the filter must be form time to time or continuously be removed inorder to avoid pressure drop over the filter. An increased pressure dropcosts fuel penalty. Therefore, particulate matter accumulated on thefilter walls at inlet side of the filter must be removed either byactive regeneration, wherein particulate matter is catalytically burnedoff in contact with an oxidation catalyst supported on the filter wallsin combination with oxygen in exhaust gas at increased exhaust gastemperatures or by non-catalytic passive regeneration.

In the passive soot regeneration the DPF is regenerated at temperaturesbelow 550° C. with NO₂ that is generated over the upstream DOC byoxidation of NO. Regeneration with oxygen in the exhaust gas should beavoided in order to control the temperature below 550° C. If the filteruncontrolled regenerates with oxygen the temperature could rise above550° C.

Despite being effective SCR catalysts, vanadium oxide based catalystscontain V₂O₅ as an essential component, which is toxic. Reports in theliterature suggest that bulk V₂O₅ has a significant vapor pressure attemperatures relevant to the catalyst operation, and both V and Wcompounds react with water to form species with increased vaporpressure.

Measurable amounts of vanadium are first released at temperatures ofabove 600° C., which is around the highest applicable workingtemperature of these systems.

Consequently, there is a risk of V and W volatile compounds can vaporizefrom the V₂O₅/WO/TiO₂ SCR catalysts in particular when integrated in theDPF. The temperature in V-SCR catalysed DPF has the highest probabilityof being exposed to temperatures exceeding 600° C., but in severe eventsthe temperature in the V-SCR catalyst can also increase above 600° C.and trigger evaporation of these compounds.

Beside the risk of emission of vanadium and tungsten compounds into theatmosphere, ammonia slip from the SCR reaction has also to beconsidered. To obtain a maximum NOx conversion, ammonia is typicallyadded to the exhaust gas in over stoichiometric amounts and unreactedammonia is emitted to the atmosphere.

The present invention seeks to solve the above problems caused byemploying vanadium and tungsten oxides as effective ammonia SCR catalystand over stoichiometric amounts of ammonia reductant in the SCR reactionin a system for the removal of particulate matter and noxious compoundsincluding nitrogen oxides from an engine exhaust gas by combining avanadium and tungsten adsorbent with an ammonia oxidation catalyst.

Thus, the present invention is in its broadest aspect a system for theremoval of volatile organic compounds, particulate matter and nitrogenoxides from exhaust gas of a compression ignition engine comprising

(a) an oxidation unit with a catalyst active in oxidation of volatileorganic compounds and carbon monoxide to carbon dioxide and water andnitrogen oxide to nitrogen dioxide;

(b) means for introducing a urea solution into the exhaust gas from unit(a);

(c) a downstream catalysed wall flow particulate filter consisting of aplurality of longitudinal inlet flow channels and outlet flow channelsseparated by gas permeable porous partition walls, each inlet flowchannel having an open inlet end and a closed outlet end, and eachoutlet flow channel having a closed inlet end and an open outlet end,the wall flow filter is catalysed with an NH₃-SCR active catalystcomprising oxides of vanadium and tungsten arranged within the gaspermeable porous partition walls and/or on the plurality of the inletand/or outlet channels of the wall flow particulate filter;(d) a downstream three way catalyst unit having an NH₃-SCR activity, anammonia oxidation activity and an adsorption capacity for volatilevanadium and tungsten compounds volatilized off the SCR active catalystof the particulate filter (c), the three way catalyst comprising highsurface compounds selected from high surface metal oxides, zeolites,silica, non-zeolite silica alumina, and mixtures thereof.

Several oxides have the property to adsorb evaporated compounds ofvanadium and tungsten. Oxides of vanadium, tungsten and titanium admixedwith at least one of a high surface ceria, alumina, silica, zirconia,non-zeolite silica alumina and zeolites, have shown as useful V and Wcompounds adsorbent and are at the same time active in the SCR reaction.These adsorbents are preferably combined with an ammonia slip catalyst(ASC).

Typical ASC formulations consist of an ammonia oxidation function basedon platinum, optionally combined with palladium on an alumina or titaniacarrier and an SCR active catalyst. In preferred formulations for use inthe invention the V,W adsorbent is applied together with an SCR catalystas a top layer on a bottom layer with the ammonia oxidation catalyst.Both layers can contain binding phases of oxide ceramics as alumina,titania, silica-alumina that have V,W adsorbing capacity.

In a specific embodiment of the invention, the three way catalystcomprises a bottom layer comprising platinum, alumina and/or titania andoptionally palladium, coated on a substrate or partly or entirelyforming the substrate, a top layer comprising oxides of vanadium,tungsten and titanium admixed with at least one of a high surface ceria,alumina, silica, zirconia, non-zeolite silica alumina and zeolite.

As the three way catalyst is arranged at the coldest position in theexhaust system any potentially evaporated V and W compounds will betrapped on the three way catalyst during the life time of the exhaustsystem on a vehicle.

High vanadium and tungsten adsorption efficiencies are achieved with arelatively thick top layer in the three way catalyst.

Thus, in preferred embodiments the top layer has layer thickness ofbetween 40 μm and 250 μm.

In further a preferred embodiment the bottom layer has a layer thicknessof between 5 μm and 80 μm. When the bottom layer itself forms partly orentirely the substrate the layer thickness is up to 450 μm.

In order to assure sufficient permeation of ammonia from the top layerto the bottom layer, the top layer must be relatively porous.

Thus, in further a preferred embodiment the top layer has a porosity ofbetween 20% and 80%.

Preferably the three way catalyst is arranged on a substrate with a flowthrough monolith shape.

When coated on a substrate in form of a flow through monolith, theamount of the top layer in the three way catalyst is between 50 and 500g per liter of the flow through monolith.

The amount of the bottom layer in the three way catalyst is preferablybetween 5 and 255 g per liter of the flow through monolith, the amountdepends on whether the bottom layer is coated on the surface of themonolith substrate or partly or entirely forms the monolith substrate.

High ammonia oxidation activities of the three way catalyst areobtained, when the bottom layer of the three way catalyst contains0.0018 g-0.35 g platinum and/or palladium per liter of the flow throughmonolith.

The top layer of the three way catalyst comprises preferably per literof the flow through monolith 1.0 g-20 g vanadium pentoxide, 3 g-40 gtungsten oxide, 40 g-460 g titania, and 0 g-86 g silica, 0 g-86 g ceria,0 g-86 g alumina, 0 g-86 g non-zeolite silica alumina and 0 g-86 g of azeolite.

Hereby it is ensured that volatile vanadium and tungsten compounds areessentially adsorbed on the surface of titania and silica and thatremaining amounts of NOx from the upstream units are selectively reducedto nitrogen and water by the SCR reaction.

In the above embodiments of the invention it is preferred that theoxidation catalyst in unit (a) upstream of the DPF comprises platinumand palladium supported on silica-alumina and/or alumina and/or titaniawith a weight ratio of platinum to palladium of 1:0 to 1:1.

The content of platinum and/or palladium in the oxidation catalyst ispreferably 0.1 g and 2 g per liter catalyst.

In still an embodiment, it may be preferred that the system comprises afurther SCR catalyst unit for decreasing remaining amounts of NOx in theexhaust gas from filter (c) by reaction with ammonia in contact with anSCR active catalyst comprising oxides of vanadium, tungsten andtitanium, the catalyst arranged between filter (c) and three waycatalyst (d).

FIG. 1 displays the NOx conversion, together with the outletconcentrations of NOx, N₂O, and N₂. The performance under theseconditions in NH₃-SCR is documented by a conversion of about 50-60% inthe temperature range of interest (250-400° C.) with a low yield of N₂Oand a high yield of N₂. FIG. 3 shows NOx conversion for NH₃-SCR andoutlet concentrations of NOx, N2, and N2O for a Pt/V—W-oxide basedmonolith three way catalyst, using a feed of 250 ppm NOx, 300 ppm NH3,12% O2, and 4% water in nitrogen at a space velocity of 100000 h⁻¹.

FIG. 2 shows the conversion of ammonia, and selectivities to N2, NOx,N2O in the selective oxidation to ammonia. In the temperature range ofinterest (250-400 □C), the ammonia is almost completely converted andthe reaction product consists mainly of nitrogen. FIG. 2 stows NH₃conversion for selective oxidation of ammonia and selectivities to NOx,N₂, and N₂O for a Pt/V—W-oxide based monolith three way catalyst, usinga feed of 200 ppm NH₃, 12% O₂, and 4% water in nitrogen at a spacevelocity of 100000 h⁻¹.

EXAMPLE 1

This example demonstrates the performance in NH₃-SCR of a three waycatalyst. The catalyst consists of Pt impregnated on a glass fiber paperbased monolith that is reinforced with TiO₂, on top of which a washcoatlayer, containing vanadium and tungsten, titanium dioxide and silica,having NH₃-SCR activity, is applied. The Pt content in the catalyst was88 mg/l. The content of the SCR active washcoat layer was 197 g/l, ofwhich 5% was silica. The catalyst was degreened at 550° C. for 1 hourprior to the performance test. The reactor feed gas consisted of 250 ppmNOx, of which less than 5% is present as NO₂, 300 ppm NH₃, 12% O₂, and4% water in nitrogen. The flow rate was adjusted to reach a SpaceVelocity of 100000 h⁻¹, based on the Monolith volume.

EXAMPLE 2

This Example Shows the Performance of the Three Way Catalyst, ascharacterized in Example 1, for selective oxidation of ammonia to reduceammonia slip. The catalyst was degreened for 1 h at 550 □C. The feed gasused in this measurement was 200 ppm NH3, 12% O2 and 4% water innitrogen. The flow was adjusted to reach a space velocity of 100000 h−1based on the monolith volume.

The invention claimed is:
 1. A system for the removal of volatileorganic com-pounds, particulate matter and nitrogen oxides from exhaustgas of a compression ignition engine comprising an oxidation unit with acatalyst active in oxidation of volatile organic compounds and carbonmonoxide to carbon dioxide and water, and nitrogen oxide to nitrogendioxide; and a catalysed wall flow particulate filter downstream of theoxidation unit, the catalysed wall flow particulate filter comprising aplurality of longitudinal inlet flow channels and outlet flow channelsseparated by gas permeable porous partition walls, each inlet flowchannel having an open inlet end and a closed outlet end, and eachoutlet flow channel having a closed inlet end and an open outlet end;wherein the system is adapted for the introduction of a urea solutioninto the exhaust gas downstream from the catalysed wall flow particulatefilter; wherein the system further comprises an SCR active catalystdownstream of the catalysed wall flow particulate filter to receiveexhaust gas into which urea solution has been introduced, the SCR activecatalyst comprising oxides of vanadium and tungsten; wherein the systemfurther comprises a three way catalyst unit downstream of the SCR activecatalyst, the three way catalyst unit having an NH₃-SCR activity, anammonia oxidation activity, and an adsorption capacity for volatilevanadium and tungsten compounds volatilized off the SCR active catalyst,the three way catalyst unit comprising compounds selected from the groupconsisting of metal oxides, zeolites, silica, non-zeolite silicaalumina, and mixtures thereof.
 2. The system of claim 1 furthercomprising a second SCR active catalyst comprising oxides of vanadium,tungsten, and titanium, the second SCR catalyst being arranged betweenthe catalyzed wall flow particulate filter and the three way catalystunit.
 3. The system according to claim 1, wherein the three way catalystunit comprises a bottom layer comprising platinum, alumina, and/ortitania, and optionally palladium coated on a substrate, and a top layercomprising oxides of vanadium, tungsten, and titanium admixed with atleast one of ceria, alumina, silica, zirconia, non-zeolite silicaalumina, and zeolite.
 4. The system according to claim 3, wherein thetop layer has layer thickness of between 40 μm and 250 μm.
 5. The systemaccording to claim 3, wherein the bottom layer has a layer thickness ofbetween 5 μm and 450 μm.
 6. The system according to claim 3, wherein thetop layer has a porosity of between 20% and 80%.
 7. The system accordingto claim 3, wherein the bottom layer of the three way catalyst unitcontains 0.0018 g-0.35 g platinum and/or palladium per liter of thesubstrate.
 8. The system according to claim 3, wherein top layer of thethree way catalyst unit comprises, per liter of the substrate, 1.0 g-20g vanadium pentoxide, 3 g-40 g tungsten oxide, 40 g-460 g titania, 0g-86 g silica, 0 g-86 g ceria, 0 g-86 g alumina, 0 g-86 g non-zeolitesilica alumina, and 0 g-86 g of a zeolite.
 9. The system according toclaim 1, wherein the three way catalyst unit comprises a substrate witha flow through monolith shape.
 10. The system according to claim 9,wherein the amount of the top layer in the three way catalyst unit isbetween 50 to 500 g per liter of the substrate.
 11. The system accordingto claim 9, wherein the amount of the bottom layer in the three waycatalyst unit is between 5 and 255 g per liter of the substrate.
 12. Thesystem according to claim 1, wherein the SCR active catalyst furthercomprises titania.
 13. The system according to claim 1, wherein thecatalyst in oxidation unit comprises platinum and/or palladium supportedon silica-alumina and/or alumina and/or titania with a weight ratio ofplatinum to palladium of 1:0 to 1:1.
 14. The system according to claim13, wherein the content of platinum and/or palladium is between 0.1 gand 2 g per liter of the catalyst.